Prosecutors for the U.S. Attorney's Office for the Southern District of New York opened the investigation following a referral from the office of special counsel Robert Mueller, the sources told NBC.

NBC previously reported that Mueller had been looking into Podesta and his Democratic-leaning lobbying firm as part of its broader investigation into the finances of former Trump campaign chairman Paul Manafort.

Tony Podesta is the chairman of the Podesta Group. His brother, John Podesta, was the campaign chairman for Hillary Clinton and is not currently affiliated with the lobbying firm. He is also not part of the investigation.